1. Field of the Invention
The present invention relates to an automatic inspection apparatus and method for detecting stains on a polarizing plate, and more particularly, to an automatic inspection apparatus and method for detecting stains on a polarizing plate, designed to objectively determine the degree of staining on a polarizing plate using color difference analysis, monitor defects in a polarizing plate in a production line in real time, and automatically inspect the quality of a polarizing plate after cutting thereof.
2. Description of the Related Art
A polarizing plate is an optical device used to allow light polarized in a certain direction to be transmitted to a liquid crystal display (LCD) device. In general, the polarizing plate is manufactured by dyeing, cross-linking and elongating a polyvinyl alcohol (PVA) film.
According to the related art, a general polarizing plate manufacturing process includes dipping and dyeing a PVA film in a solution containing iodine or dye, cross-linking the iodine or dye with the PVA film by adding boric acid or the like thereto, and elongating the PVA film. The dyeing, cross-linking and elongating processes may be conducted consecutively or simultaneously, and the order of these individual processes may also be varied. After the PVA film is completely subjected to the dyeing, cross-linking and elongating processes, it may be dried to thereby form a PVA polarizer. A protective film such as a triacetyl cellulose (TAC) film is attached to one surface or both surfaces of the PVA polarizer using a PVA adhesive or the like, and thus, a polarizing plate is manufactured.
However, striped stains may be present on the polarizing plate manufactured as described above in a machine direction (MD) thereof due to uneven dyeing, poor attachment, or the like. When stains are severely formed thereon, the brightness of a screen is not uniform, and thus, final product defects may result therefrom. Therefore, a sorting operation is required to pick out defective products by measuring the degree of staining on polarizing plates. In general, the inspection of stains on the polarizing plates is carried out with the naked eye by human inspectors. However, this inspection method has problems in that it may be difficult to produce products having uniform product quality since the degree of defects in final products is subjectively determined by the inspectors.
Accordingly, in recent years, a method of quantifying the degree of staining on a polarizing plate has been sought. As a result, the following inspection apparatus and method have been provided: arranging a target polarizing plate to be disposed between reference polarizing plates having parallel absorption axes while an absorption axis of the target polarizing plate is perpendicular to those of the reference polarizing plates; quantifying the degree of staining on the target polarizing plate using intensity (contrast) data from among image data obtained by imaging the target polarizing plate while being irradiated with light; and objectively inspecting the stains on the target polarizing plate.
In such an inspection method, however, since the absorption axes of the reference polarizing plates and the target polarizing plate are perpendicular to each other, an amount of light transmitted through the target polarizing plate is so small that an exposure time of an imaging apparatus should be relatively lengthened in order to obtain analyzable image data. An excessive exposure time may cause a difference between stains obtained from RGB data extracted from the image data and stains observed with the naked eye by inspectors. Further, according to the related art, stains are quantified by utilizing intensity data from among the extracted image data, and accordingly, it is difficult to verify stains caused by a delicate color difference.
In order to solve the above-mentioned problems, developments in the quantification of stains on a polarizing plate and automatic inspection apparatus and inspection method are required.